High-energy charged particles are used today for many purposes. General areas of application are e.g. medical treatment, sterilization and material modification. Common to all these methods is that charged particles have to be accelerated under controlled conditions to high energies.
In the field of particle accelerator devices, high voltages are the most common means to obtain accelerated charged particles. Charged particles, in most cases electrons, are emitted from a particle source, usually a filament. The particles are subjected to the field of a high-voltage difference, and are thereby accelerated. The acceleration usually takes place in a vacuum environment, and in applications where irradiation by the charged particles are to be performed under atmospheric pressure, the charged particles are allowed to penetrate radiation windows to escape into the atmospheric environment.
There are two general approaches to achieve the high particle energies. The straight-forward approach is to achieve a high voltage, preferably by a transformer. A relatively moderate voltage at the primary winding of the transformer is transformed to a high voltage at the secondary winding, which voltage can be used for accelerating the charged particles. The most common way is to use an ordinary transformer with an iron core. However, when the voltage rises above 100 kV, the insulation problems become severe.
Another approach is therefore often used for producing the high particle energies. This approach is based on microwave excitation. Such methods are generally expensive and require a lot of complicated and bulky equipment.
A common problem with the above methods according to the state of the art is that the acceleration devices are large and expensive, which makes it impossible to use them in a machine located in a standard production line for most purposes.
There are several proposals for overcoming the limitations of the transformer approach. Since the beam of particles normally is pulsed, the energy transformation in the transformer may utilize resonant behaviors of the equipment. The U.S. Pat. No. 3,450,996 discloses an accelerator device including a Tesla coil transformer. The primary circuit of the Tesla transformer comprises the primary winding and a capacitor, over which the primary voltage is applied. The primary circuit has a certain resonant frequency. A switch controls the current flowing through the primary winding. The secondary circuit comprises the secondary winding, stray capacitances and the load, all connected in parallel. The secondary circuit also has a resonant frequency, which is tuned to be identical with the resonant frequency of the primary circuit.
When closing the switch, the voltage over the primary capacitance will give rise to a current through the primary winding. The current in the primary circuit gives rise to an electromagnetic field, which in turn induces a current in the secondary winding. A voltage over the load in the secondary circuit will eventually build up. The resonant behavior efficiently transfers energy between the primary and secondary circuits. When the peak voltage over the load in the secondary circuit is reached, a short pulse of high-energy particles can be produced. The rest of the energy in the double resonance circuitry is collected back in the primary circuit, the switch is opened and the voltage over the primary capacitance is allowed to build up again.
According to prior art, the method works well in theory, but gives rise to many problems when applying it into practice, at least for very high voltages. A very high voltage on the secondary side requires a very high ratio between the number of turns in the primary and secondary circuits. A huge number of secondary turns is not easily achievable, so the number of primary turns has to be limited. However, a turns ratio above 100 is not easy to achieve according to the prior art. This means, for instance, that if a final secondary voltage of above 1 MV is required, the voltage of the primary side has to be of the order of 10 kV.
The insulation problems become severe, and an ordinary iron core design can not be used. In the patent U.S. Pat. No. 3,450,996, a magnetic conductor is disposed outside the primary circuit, in order to insulate it from the high voltages of the secondary circuit.
In order to operate the transformer of U.S. Pat. No. 3,450,996, the switch has to be operable at high voltages, both for opening and for closing. If the pulse duration is short, this opening and closing has to be performed very accurately and fast. For handling voltages up to 10 kV, thyristor devices have to be used. However, the opening times and precision for such equipment are limited, Furthermore, the devices have to recover after an opening before they can be closed again. This makes it necessary to incorporate complicated circuitry to accomplish the required high frequency switching.
During recent years, the technology of IGBT (Integrated Gate Bipolar Transistor) has provided electronically controlled high-voltage switches, which can accomplish both relatively fast turn-on and turn-off with high precision. However, today, the IGBT is limited to a maximum voltage of about 2 kV, which makes them unsuitable for applications of very high-voltages. One solution would in theory be to stack a number of IGBTs on top of each other, and control the turn-on and turn-off simultaneously. However, when dealing with turn-on and turn-off times in the order of microseconds, the synchronization becomes a severe problem. If the time when each of the IGBTs is turned on is not the same, the total voltage over the stack will be placed over the last IGBT to be turned on, which is likely to lead to the destruction of this component.
Devices for producing short pulses of high-voltage according to prior art are therefore expensive, bulky and require extremely complicated control electronics.